The present invention pertains to an improved transconductance amplifier circuit.
A transconductance amplifier circuit provides an output current that is proportional to its input voltage.
One prior art transconductance amplifier circuit which provides a precise relationship between the output current and the input voltage is shown in FIG. 1. This circuit includes an operational amplifier A; a D.C. voltage source V completing the negative feedback loop between output terminal of the operational amplifier A and inverting input terminal of the operational amplifier A; and a resistance R completing the positive feedback loop between the output terminal of the operational amplifier and the non-inverting input terminal of the operational amplifier A. An operational amplifier characeristically has a sufficiently high input impedance that any current flow at its input terminals is effectively negligible. Accodingly there is in effect no current flow between the current output terminal I and the non-inverting input terminal of the operational amplifier A, whereby the amount of current i.sub.out flowing through the load resistance R.sub.L to circuit ground potential is proportional to the voltage V. i.sub.out = V/R. The operational amplifier A must include an output stage containing a current amplifier capable of delivering a current of the desired magnitude and in accordance with the magnitudes of the circuit components V and R. With a battery provided as the voltage source V, the output current terminal I is in effect a constant current source.
The prior art circuit of FIG. 1 is somewhat limited in utility in that the voltage source V must float above circuit ground and it must be dedicated (i.e. the voltage source cannot be used for any other purpose within an overall larger circuit including the transconductance amplifier circuit of FIG. 1 as a component). The preciseness of the constant current source at output current terminal I is dependent upon the preciseness of the battery dedicated to the voltage source V. Also there is a capacitive effect between the voltage source V and circuit ground which limits the frequency response of the circuit of FIG. 1.
Another prior art transconductance amplifier circuit is shown in FIG. 2. The inverting input terminal of an operational amplifier B is connected to ground potential through a resistance RA. A resistane RB connects the input voltage source E to the non-inverting input terminal of the operational amplifier B. The negative feedback loop of the operational amplifier B is completed by a resistance RC; and the positive feedback loop is completed by the resistance RD. The output current i.sub.out is provided from the junction of the resistances RB and RD. For this circuit when RD.multidot.RD = RA.multidot.RD then: EQU i.sub.out = E (RA.multidot.RD/RA.multidot.RB.multidot.RD).
with this transconductive amplifier circuit the voltage source E must have the capability of the delivering of a current of the desired magnitude. Also in order to change the scale factor defining the relationship between the output current i.sub.out and the input voltage E it is necessary that means be provided for changing the values of both resistance RB and resistance RD.
Other prior art transconductance amplifier circuits include current to voltage transformers in their output stages to sense current flow and then convert the sensed current to a voltage signal so as to provide a feedback signal. These cicuits are generally quite complicated and not particularly accurate, and are capable of responding to only AC input voltage signals.